The present invention relates to an optical information recording medium on which information can be recorded, reproduced, erased and rewritten with high density and at high speed using an optical means, such as irradiation of a laser beam.
In an optical information recording medium, the difference in optical characteristics caused by local irradiation of a laser beam onto a recording material is utilized as a recording state. When using a material whose optical characteristics are varied reversibly, erasing and rewriting of information is possible. As rewritable media, generally a magneto-optical recording medium and a phase change recording medium have been well known. In these optical recording media, a large volume of information can be recorded, and recording, reproduction, erasure, and rewriting can be performed at high speed. In addition, such optical recording media are excellent in portability. Therefore, it is conceivable that in a highly information-oriented society, the demands for such optical recording media will further increase, and thus it is desired to increase further their capacity and the speed in recording, reproducing, erasing or rewriting information in them.
In the phase change recording medium, with respect to light with a specific wavelength the quantity of reflected light from a portion in a crystalline state is different from that from a portion in an amorphous state, and this difference is utilized as a recording state. By modulating an output power of the laser, erasure of recorded signals and recording by overwriting can be performed at the same time. Thus, information signals can be erased and rewritten easily at high speed.
FIG. 9 shows an example of a layer structure of a conventional phase change recording medium. As shown in FIG. 9, the conventional phase change recording medium includes a substrate 1, and a protective layer 2, a recording layer 4, a protective layer 8, and a reflective layer 6 that are laminated sequentially on the substrate 1. As the substrate 1, resin such as polycarbonate or PMMA, glass, or the like is used. In the substrate 1, a guide groove for guiding a laser beam is formed. The recording layer 4 has states different in optical characteristics and is formed of a material that can be varied reversibly between the different states. In the case of a rewritable phase change optical recording material, generally a chalcogenide-based material containing Te or Se as the main component is used as the material for the recording layer 4. Examples of the chalcogenidexe2x80x94based material include a material containing Texe2x80x94Sbxe2x80x94Ge, Texe2x80x94Snxe2x80x94Ge, Texe2x80x94Sbxe2x80x94Gexe2x80x94Se, Texe2x80x94Snxe2x80x94Gexe2x80x94Au, Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te, Inxe2x80x94Sbxe2x80x94Se, Inxe2x80x94Texe2x80x94Se, or the like as the main component. Generally, the reflective layer 6 is formed of metal such as Au, Al, Cr, or the like or an alloy thereof and is provided for the purpose of obtaining a heat release effect and effective optical absorption in the recording layer 4. In addition, for the purpose of preventing oxidation and corrosion of the optical information recording medium or adhesion of dust thereon, a configuration with an overcoat layer on the reflective layer 6 or a configuration in which a dummy substrate is laminated using ultraviolet curing resin as an adhesive is used in general, although it is omitted in the figure. The protective layers 2 and 8 have functions for protecting the recording layer 4, such as functions for preventing oxidation, evaporation, or deformation of a material of the recording layer 4. By adjusting the thickness of the protective layers 2 and 8, the absorptance of the recording medium or the difference in reflectance between a recording portion and an erasure portion (hereinafter referred to as xe2x80x9creflectance differencexe2x80x9d) can be adjusted. Therefore, the protective layers 2 and 8 also have a function for adjusting optical characteristics of the recording medium. As conditions of the material forming the protective layers 2 and 8, it is necessary that not only the above-mentioned purposes are attained but also excellent adhesiveness between, for example, the material of the recording layer 4 and the substrate 1 can be obtained and the protective layers 2 and 8 themselves are films with excellent weather resistance in which no cracks occur. Further, when the protective layers 2 and 8 are used while being in contact with the recording layer 4, the material of the protective layers 2 and 8 should be one that does not hinder the optical change of the material of the recording layer 4. As a material of the protective layers 2 and 8, in addition to sulfide such as ZnS or the like, oxide such as SiO2, Ta2O5, Al2O3, or the like, nitride such as Gexe2x80x94N, Si3N4, Al3N4, or the like, or nitrogen oxide such as Gexe2x80x94Oxe2x80x94N, Sixe2x80x94Oxe2x80x94N, Alxe2x80x94Oxe2x80x94N, or the like, dielectric such as carbide, fluoride, or the like, or suitable combinations thereof have been proposed. Generally, ZnSxe2x80x94SiO2 has been used in many cases.
Conventionally, it has been known that when recorded signals are erased and rewritten, mark positions after rewriting are shifted slightly and overwriting distortion occurs. This distortion occurs because the manner of temperature rise when a laser beam is irradiated is different depending on whether the state of the recording layer 4 before rewriting is in an amorphous state or in a crystalline state, and thus rewritten marks have different lengths from predetermined lengths. In other words, when a mark is in an amorphous state, a latent heat is required for the phase change into an amorphous state at portions that were in a crystalline state before rewriting, but it is not required at portions that were in an amorphous state before rewriting. Therefore, an excess amount of heat changes a portion longer than a predetermined length in the recording layer 4 into an amorphous state. In order to solve this problem, a configuration is employed in which Ac/Aa is maintained to be larger than 1 and in a certain range, wherein Aa indicates optical absorptance of the recording layer 4 in an amorphous state and Ac represents optical absorptance of the recording layer 4 in a crystalline state. In other words, a configuration that enables so-called absorptive correction is employed. This promotes temperature increase at a crystalline portion and therefore a uniform temperature rise within a mark region after rewriting can be obtained. Thus, in this case, mark distortion does not occur easily.
As a method of attaining Ac/Aa greater than 1, some methods have been proposed. For example, a configuration (satisfying Rc less than Ra) in which the reflectance Ra of a portion in an amorphous state is set to be higher than the reflectance Rc of a portion in a crystalline state has been proposed. In this case, a high value of Ac/Aa can be obtained even when the reflectance difference Raxe2x88x92Rc between the portion in the amorphous state and the portion in the crystalline state is set to be large. Concretely, for instance, Rc less than Ra can be attained by providing another layer between the substrate 1 and the protective layer 2 in FIG. 7 and setting the optical constant of the another layer within a certain range.
Further, even in the case of Rc greater than Ra, Ac/Aa greater than 1 can be attained. As methods for attaining this, those mainly using an optical transmission type medium and an optical absorption type medium have been known. The optical transmission type medium is used in a method in which the medium is allowed to have transmittance and a configuration satisfying 0 less than Tc less than Ta is employed, wherein Ta indicates transmittance of the medium when its recording layer is in an amorphous state and Tc represents transmittance of the medium when its recording layer is in a crystalline state. The optical absorption type medium is used in a method in which a layer causing absorption is provided in the medium and a configuration satisfying 0 less than Ac2 less than Aa2 is employed, wherein Aa2 indicates optical absorption in this layer when the recording layer is in an amorphous state and Ac2 represents that when the recording layer is in a crystalline state. Concretely, in the case of the optical transmission type medium, Ac/Aa greater than 1 can be attained by, for instance, reducing the thickness of the reflective layer 6 in FIG. 9 and enabling optical transmission. In the case of the optical absorption type medium, for example, Ac/Aa greater than 1 can be attained by, for example, inserting a layer absorbing light between the reflective layer 6 and the protective layer 8 in FIG. 9.
As described above, a medium with a configuration satisfying a reflectance relationship of Rc less than Ra has a great advantage in that a configuration satisfying Ac/Aa greater than 1 can be designed easily. However, the sum of reflectance of the portions in an amorphous state and in a crystalline state is generally larger than that in a medium having a configuration satisfying a reflectance relationship of Rc greater than Ra. Therefore, there is a disadvantage in that noise in reproducing signals increases easily. In the case of a medium with a configuration satisfying a reflectance relationship of Rc greater than Ra, such a disadvantage is not caused easily. However, the medium has a disadvantage in that a large value of Ac/Aa cannot be obtained. Therefore, it is desirable to use these methods properly depending on required media.
With respect to the configuration of the optical transmission type medium satisfying Re greater than Ra and 0 less than Tc less than Ta, conventionally some improvements have been proposed.
For example, in JP-A-8-050739, a technique is disclosed in which a recording layer and an optical transmission type reflective layer are provided and a thermal diffusion auxiliary layer for helping thermal diffusion in the reflective layer is provided while being contact with the reflective layer. This publication, however, does not describe about a technique for providing optical effects to the thermal diffusion auxiliary layer actively, and it is described that the thickness of the layer is set to be in a range in which optical design is not hindered. In JP-A-9-91755, a technique of providing a dielectric layer on an optical transmission type reflective layer is disclosed. In this case, however, the dielectric layer is provided for reducing phase difference and there is no description about thermal effects obtained by providing the dielectric layer. In addition, the publication also does not describe the optical effects obtained by adjusting the thickness of the layer.
As an example in which optical transmission type media are applied, a so-called multilayer recording medium technique has been known. This technique enables access to all recording media by providing at least two sets of recording media via a transparent separating layer and allowing a laser beam to be incident only from one side. By using this technique, the recording density in an incident direction of a laser beam can be increased and the capacity of the multilayer recording medium as a whole can be increased.
The optical transmission type configuration is advantageous in repeatability or adjacent erasing characteristics due to a relatively small amount of excess heat filling the inside of the medium. However, since the reflective layer is thin, it is difficult to cool a recording layer quickly after the layer was heated and therefore marks are difficult to be formed, which has been a problem. Furthermore, particularly in the case of a configuration satisfying Rc greater than Ra, basically it was difficult to have a very high value of Ac/Aa. In designing an optical transmission type medium to be positioned on an incident side of a laser beam for forming a multilayer recording medium, conventionally it was required to decrease the thickness of a recording layer to obtain sufficiently high transmittance. However, when the recording layer is very thin, crystallization occurs with difficulty. Consequently, it was difficult to obtain the compatibility between high transmittance and a high erasing ratio. There is no example considering a technique for further improving repeated recording characteristics of an optical transmission type medium. New techniques for further improving the repeated recording characteristics have been demanded.
The present invention aims to solve the above-mentioned problems in the conventional technique, to provide an optical information recording medium in which the cooling power of the recording medium can be improved, distortion of overwritten marks can be decreased, and recording can be performed at a higher speed with higher density, and to provide optical transmission type recording media for a multilayer recording medium in which the compatibility between high transmittance and a high erasing ratio can be obtained.
In order to attain the above-mentioned objects, a configuration of an optical information recording medium according to the present invention includes: a recording layer whose optical characteristics are varied reversibly by irradiation of a laser beam; a reflective layer that transmits the laser beam with a wavelength xcex; and a thermal diffusion layer provided while being in contact with the reflective layer. The configuration is characterized in that a thickness d of the thermal diffusion layer is within a range of 0 less than dxe2x89xa6({fraction (5/16)})xcex/n or ({fraction (7/16)})xcex/nxe2x89xa6dxe2x89xa6(xc2xd)xcex/n, wherein n indicates a refractive index of the thermal diffusion layer. According to the configuration of this optical information recording medium, the cooling power of the recording layer can be further improved and at the same time the overwriting distortion can be decreased due to the improvement in value of Ac/Aa. Therefore, recording can be performed at higher speed with higher density.
In the configuration of the optical information recording medium according to the present invention, it is preferable that the thermal conductivity of a material forming the thermal diffusion layer is at least 0.05 W/mxc2x7K at 500K. According to this preferable example, the cooling effect in the thermal diffusion layer can be further improved.
In the configuration of the optical information recording medium according to the present invention, it is preferable that a refractive index of the thermal diffusion layer is at least 1.6 with respect to a wavelength of a laser beam used for recording and reproduction of information. According to this preferable example, the effect for increasing the value of Ac/Aa in the thermal diffusion layer can be made more effective.
In the configuration of the optical information recording medium according to the present invention, it is preferable that an absorption coefficient of the thermal diffusion layer is 1.5 or less with respect to a wavelength of a laser beam used for recording and reproduction of information. According to this preferable example, heat generation in the thermal diffusion layer can be further suppressed, thus allowing the cooling effect in the thermal diffusion layer to be more effective.
In the configuration of the optical information recording medium according to the present invention, it is preferable that the thermal diffusion layer contains at least one selected from a group consisting of Alxe2x80x94N, Alxe2x80x94Oxe2x80x94N, Alxe2x80x94C, Si, Sixe2x80x94N, SiO2, Sixe2x80x94Oxe2x80x94N, Sixe2x80x94C, Tixe2x80x94N, TiO2, Tixe2x80x94C, Taxe2x80x94N, Ta2O5, Taxe2x80x94Oxe2x80x94N, Taxe2x80x94C, Znxe2x80x94O, ZnS, ZnSe, Zrxe2x80x94N, Zrxe2x80x94Oxe2x80x94N, Zrxe2x80x94C, and Wxe2x80x94C.
In the configuration of the optical information recording medium according to the present invention, it is preferable that the reflective layer contains at least one selected from a group consisting of Au, Ag, and Cu. According to this preferable example, the value of Ac/Aa can be set to be large and at the same time high cooling power can be obtained due to high thermal conductivity even in the case where the reflective layer is thin.
In the configuration of the optical information recording medium according to the present invention, it is preferable that the reflective layer has a thickness between 1 nm and 20 nm. When the reflective layer is thinner than 1 nm, it is difficult to form it so as to be a uniform layer. Therefore, thermal and optical effects of the reflective layer decrease. In the case where the reflective layer is thicker than 20 nm, the light transmittance of the medium decreases and therefore it is difficult to obtain the optical absorptive correction (Ac/Aa greater than 1).
In the configuration of the optical information recording medium according to the present invention, it is preferable that the recording layer has a thickness between 3 nm and 20 nm. When the recording layer is thinner than 3 nm, it is difficult to form a recording material into a uniform layer. Therefore, effective phase change between an amorphous state and a crystalline state is difficult to ensure. In the case where the recording layer is thicker than 20 nm, the thermal diffusion within an inplane of the recording layer increases, thus causing adjacent erasure easily in performing recording with high density.
In the configuration of the optical information recording medium according to the present invention, it is preferable that the recording layer is formed of a phase change material containing at least one selected from a group consisting of Te, Se, and Sb.
In the configuration of the optical information recording medium according to the present invention, it is preferable that the optical information recording medium has an average light transmittance between 40% and 80% with respect to the laser beam, further preferably between 50% and 70%. In this case, the average light transmittance is defined as the transmittance in a state where signals have been recorded in the medium (hereinafter, the average light transmittance is referred to simply as xe2x80x9clight transmittancexe2x80x9d). According to this preferable example, when in the medium, another recording medium is provided on the opposite side to an incident side of a laser beam, recording and reproduction in both the media can be performed only by the irradiation of the laser beam from one side. It is highly preferred to employ a configuration of this so-called multilayer recording medium, since the recording capacity of the medium can be increased efficiently.
Moreover, in this case, it is preferable that at least one other optical information recording medium is provided on the side opposite to an incident side of the laser beam. According to this preferable example, further a high-density medium can be obtained.
In the configuration of the optical information recording medium according to the present invention, it is preferable that an interface layer having an effect for facilitating crystallization of the recording layer is provided while being in contact with at least one side of the recording layer.
Particularly when an optical transmission type medium is designed so as to have high light transmittance, a recording layer becomes very thin and therefore the crystallization of the recording layer is difficult in many cases. However, when the interface layer is provided while being in contact with the recording layer, it is possible to shorten the time required for the crystallization of the material of the recording layer, thus enabling recording at higher speed.
Further, in this case, it is preferable that the interface layer is formed of a material containing at least N. A material containing N is excellent in denseness, thus greatly shortening the time required for the crystallization of the material of the recording layer.